I think you are using the wrong word when you say multimeter, you can measure the power with one but you  have to make two measurements, the voltage and then the current and then multiply them together to get power in watts.
Are you using a meter that plugs into a wall socket and then you plug the lead for the mining rig into the meter? Thats a wattmeter, or Killawatt.
The efficiency of the psu is significant. With a 80% psu there is a 20% difference from input to output. So adding up the gpus and cpu and other system power will always be 20% lower than the power measured on the mains input. If all the gpus and stuff added up to say 1000 watts, the input power would read as 1200, 20% higher.

Baz